1. Field of Invention
The present invention generally relates to a light emitting diode (LED) lighting device.
2. Related Art
When a plurality of light emitting diodes (LEDs) of the same color or different colors are connected in series into several series of LEDs, it is likely to have different VF values in individual LEDs (1.8 V to 2.1 V for red ones and 3.6 V to 4.0 V for blue and green ones) which result in uneven electric currents. This phenomenon of uneven electric currents would damage LEDs with larger currents flowing through and make LEDs with smaller currents unable to have sufficient luminosity.
Since the relation between the luminosity of the LED and the electric current flowing through is not linear. In the prior art, the control of the LED luminosity can only be achieved by changing the duty cycle of lighting the LEDs. This technology has been widely used in LED signs for over 20 years and become a fairly mature technology.
The above-mentioned control can be achieved by a software program performed on a microprocessor, a digital logic circuit, or a devoted integrated circuit (There are already IC factories designing and producing this type of ICs for LED signs). A PWM logic circuit 120 composed of digital devices can increase the frequency of the lighting cycle of the LED. The software control through a microprocessor can only go up to 350 Hz (i.e. a repetition rate of 350 times per second). This frequency can not be perceived by naked eyes. However, from a camera one can apparently see the blinking. This blinking can even be observed by a relative motion between the eyes and the lighting unit. Thus, it is bad for image production.
When a large number of LEDs are located together and arranged in one direction in a usual way, the emitted light would have speckles (i.e. uneven color mixing). The wavelength and luminosity of the light emitted from the LED of a specific color are limited by the LED production technology to certain ranges. If an equal number of red, green, and blue LEDs are employed, different electric currents have to provided for LEDs of different colors so as to obtain a light spectrum similar to the natural light. Accordingly, in the long run, LEDs with larger currents flowing through decay faster in the luminosity whereas LEDs with smaller currents flowing through decay more slowly.
Moreover, the LED also generates heat when emitting light. There is no problem in heat dissipation for a single LED. Since the luminosity of a single LED is not high enough, many LEDs have to be collected for a substantial operation power. If so, the heat generated by the LEDs would be harmful to themselves. If the heat generated by the LEDs can not be dissipated immediately, the density of LEDs can not be increased to reduce the device volume and its lifetime may be shortened.